Thermochromic materials undergo color changes in response to thermal exposure. This is frequently discussed in context of a hysteresis plot where color is activated and deactivated by thermal cycling. This hysteresis is shown, for example, in U.S. Pat. No. 4,442,429 issued to Kotani et al. The materials may be designed for specific applications to have little thermal separation between the different sides of the plot, or a large separation. Other aspects of performance design may include the thermal activation temperature.
Thermochromic pigments are conventionally made by well-known manufacturing processes. Typically, a leuco dye, a developer, oil, and a polymer, such as melamine formaldehyde or urea formaldehyde, are combined and agitated to create a very fine emulsion. The properties of the emulsion are such that the oil dye, and developer reside within a polymer formaldehyde capsule. Melamine formaldehyde is a thermo set resin similar to formica. The substance is very hard and will not break down at high temperature. This material is almost entirely insoluble in most solvents, but it is permeable. Thus, U.S. Pat. No. 5,591,255 issued to Small et al. advances the art by advising to forego use of additives that may permeate the capsules, especially certain ketones, diols, aldehydes, amines and aromatic compounds.
Even so, emerging new applications for thermochromic pigments demand the use of additives that interfere with functionality of the thermochromic pigments. By way of example, uses are emerging for thermochromic pigments that may be mixed with epoxy or polyester vehicles to form metal deco coatings. If desired, as suggested by U.S. Pat. No. 4,425,161 issued to Shibahashi et al., the thermochromic pigments may be used in combination with such polymers as, hydrocarbon resin, acryl resin, vinyl acetate resin, halogen-containing resin, diene resin, polyester resin, polyamide resin, polyurethane resin, epoxy resin, melamine resin and polyurea resin. This may necessitate the addition of auxiliary solvents including such diluents as high boiling point aromatic hydrocarbon solvents, waxes, terpene oils and fluorocarbon oils, as well as solvents to improve the rheology of the coatings. Thermochromic pigments blended with these materials have typically a short shelf life. Moreover, the materials in these vehicles may significantly alter the thermochromic behavior of the pigments, such as by suppressing the color activation temperature or varying the width of the hysteresis window.
Known processes for microencapsulation of thermochromic or photochromic materials include, for example, those disclosed in U.S. Pat. No. 4,028,118 issued to Nakasuji, U.S. Pat. No. 4,425,161 issued to Kito et al., U.S. Pat. No. 4,425,161 issued to Shibahashi et al., U.S. Pat. No. 4,902,604 issued to Yamaguchi et al., U.S. Pat. No. 5,350,633 issued to Sumii et al., U.S. Pat. No. 5,503,781 issued to Sumii et al., U.S. Pat. No. 6,139,779 issued to Small et al., and U.S. Pat. No. 7,732,109 issued to Senga et al., al. of which are incorporated by reference to the same extent as though fully disclosed herein.